Assessment of the accuracy and stability of frameless gamma knife radiosurgery

Abstract

The aim of this study was to assess the accuracy and stability of frameless gamma knife radiosurgery (GKRS). The accuracies of the radiation isocenter and patient couch movement were evaluated by film dosimetry with a half-year cycle. Radiation isocenter assessment with a diode detector and cone-beam computed tomography (CBCT) image accuracy tests were performed daily with a vendor-provided tool for one and a half years after installation. CBCT image quality was examined twice a month with a phantom. The accuracy of image coregistration using CBCT images was studied using magnetic resonance (MR) and computed tomography (CT) images of another phantom. The overall positional accuracy was measured in whole procedure tests using film dosimetry with an anthropomorphic phantom. The positional errors of the radiation isocenter at the center and at an extreme position were both less than 0.1 mm. The three-dimensional deviation of the CBCT coordinate system was stable for one and a half years (mean 0.04 ± 0.02 mm). Image coregistration revealed a difference of 0.2 ± 0.1 mm between CT and CBCT images and a deviation of 0.4 ± 0.2 mm between MR and CBCT images. The whole procedure test of the positional accuracy of the mask-based irradiation revealed an accuracy of 0.5 ± 0.6 mm. The radiation isocenter accuracy, patient couch movement accuracy, and Gamma Knife Icon CBCT accuracy were all approximately 0.1 mm and were stable for one and a half years. The coordinate system assigned to MR images through coregistration was more accurate than the system defined by fiducial markers. Possible patient motion during irradiation should be considered when evaluating the overall accuracy of frameless GKRS.

Keywords: frameless radiosurgery; gamma knife; image coregistration; positional accuracy; whole procedure test.

Figure 1

(a) Gamma Knife Icon^TM and its CBCT system in an imaging setup. (b) A vendor‐provided tool was used to measure deviations of the radiation isocenter and the accuracy of CBCT images. The white arrow indicates the diode detector used to measure dose distributions, and the dotted arrow indicates one of the four ball bearings employed to assess the geometrical accuracy of the CBCT images. (c) A CatPhan^® 503 phantom was used for CBCT image quality assessment.

Figure 2

Examples of the CIRS 603a phantom images used for image coregistration error analysis. (a) GK Icon CBCT. (b) GE Discovery CT. (c) GE Signa 1.5 T MR.

Figure 3

An anthropomorphic phantom is fixed to the GK Icon with a mask system for a whole procedure test of the accuracy of frameless GKRS.

Figure 4

An in‐house device used to assess HDMM accuracy. The circular marker on the cube is the patient marker used to check patient movement. The cube was moved by a depth micrometer along a straight line guided by two plastic rulers.

Figure 5

Deviations between the radiation iso‐center and the center of the patient positioning system. Radiochromic films were irradiated using a 4‐mm collimator at the center and at an extreme position (40, 160, 100).

Figure 6

Variations of the mean one‐ and three‐dimensional deviations obtained for locations of the four ball bearings in the GK Icon CBCT.

Figure 7

Variations of the contrast to noise ratio (CNR) and uniformity of the GK Icon CBCT images over time.

Figure 8

Relationship between the one‐dimensional motion of the micrometer and the movement measured by the HDMM system.